In recent years there has been great progress in the understanding of the genetic basis of many medical disorders including neuropsychiatric disorders such as those associated with mental retardation. While the genetic defects of such disorders are disparate and each mechanism appears to be novel, the unifying theme behind the development of these diseases is cellular development and maintenance of cellular networks. For example, for neuropsychiatric disorders the development of these diseases results in part from abnormal brain development and maintenance of neuronal networks.
Neurological networks are made up of individual neurons, each neuron being a separate structural and functional cellular unit. Neurons have special features for the reception of nerve impulses from other neurons, the effect of which may be either excitation or inhibition, and conduction of nerve impulses. Neurons commonly have long cytoplasmic processes known as neurites which end in close apposition to the surfaces of other cells. The ends of these neurites are called synaptic terminals and the cell-to-cell contacts they make are known as synapses. The neurites in higher animals are usually specialized to form dendrites and axons which conduct impulses toward and away from the cell body, respectively. The arrival of an impulse at a terminal triggers the process of synaptic transmission. This event usually involves the release of a chemical compound from the neuronal cytoplasm invoking a response in the postsynaptic cell. Neurons of the central nervous system consist of discrete segments including the cell body, the dendrites and the axon. While most nerve cells conform to this basic structure, there is a wide range of structural diversity based upon the specific function of the cell within the body.
It has been shown that these polarized cells contain a variety of cytoplasmic and membrane-bound proteins differentially distributed throughout the axon, dendrites, and cell body of the neuron. It is believed that neurons of the central nervous synthesize proteins locally, at or near postsynaptic sites which are independent of the cell body. Ultrastructural studies have revealed that polyribosomes are preferentially located either beneath post-synaptic sites or occasionally associated with membrane specializations on dendrites. It has been suggested that these anatomical structures represent the protein synthetic machinery necessary to translate and post-translationally modify different classes of protein in neurons. An energy-dependent mechanism for the selective transport of RNA in neurons has also been shown. The nature and distribution of the RNAs present in these cells, however, is poorly understood.
In situ hybridization (ISH) studies have been successful in identifying very few mRNAs in neuronal processes. Studies using in situ hybridization and Northern blot analyses of synaptosomal RNA fractions with the AMPA-GluR1, -GluR2, GluR3 and GluR-4 and kainate-sensitive GluR5 and GluR6 receptor subunits failed to reveal mRNAs at dendritic locations. (Craig, A. et al., Neuron 1993, 10, 1055-1068; Chicurel, M. et al. J. Neurosci 1993, 13, 4054-4063).
Microdissection of individual neurites has now revealed a large number of mRNAs, including members of the glutamate receptor family, second messenger components, and components of the translational control apparatus, present in hippocampal neurites.
It has now been found that the profiles of expressed mRNAs from discrete segments of the same neurons have different characteristics. These differences in expressed mRNA can be used as a means to specifically target discrete segments of the neuron and to identify and diagnose genetic neurological disorders at the molecular level. Characterization of multiple mRNAs in single cells can also be useful in diagnosis and treatment of neurological disorders and other diseases.